The present invention relates to a method for preparation of a thermoplastic resin composition with good impact-resistance and, in particular, to a method for preparation of a thermoplastic resin composition containing two different crosslinked rubbers uniformly dispersed in a polyolefin phase, preferably polypropylene.
A thermoplastic elastomeric composition comprising a polyolefin and a partially crosslinked rubber component has excellent properties, such as heat-resistance, mechanical strength, softness and elasticity, and such compositions are being widely utilized for formation of large-sized shaped articles such as bumpers for cars.
Regarding thermoplastic elastomers of this type, JP-B-No. 53-34210 (the term "JP-B" as used herein means an "examined Japanese patent publication") illustrates a thermoplastic blend formed by dynamically partially curing from 60 to 80 parts by weight of a monoolefin copolymer rubber and from 40 to 20 parts by weight of a polyolefin plastic. The thermoplastic blend may be obtained by mixing the said components together with a curing agent and dynamically mixing them at the curing temperature to cure the same.
In addition, there were proposed, for example, a partially crosslinked composition comprising a polyolefin and two kinds of rubber compositions (JP-B-No. 54-23702), a partially crosslinked composition obtained by incorporating a polyolefin into a composition formed by processing a polyolefin and a rubber component in the presence of an organic peroxide (JP-B-No. 56-15743), and a method for preparation of the said composition (JP-B No. 56-15740).
However, organic peroxides to be used in the preparation of the above-mentioned partially crosslinked thermoplastic elastomer composition as a crosslinking agent are too strongly reactive so that it is difficult to control the crosslinking degree in the composition. Moreover, there are still other problems that the physical properties of the crosslinked composition would be reduced because of the cleavage of molecular chains following crosslinking in the composition. The resulting low molecular weight substances would have a bad influence on the coatability of the composition, and additionally the shaped article from the composition would have a rough surface because of the partial gelation in the composition. In addition, free radicals may be present in the shaped article because of the organic peroxide used so that the heat stability of the article would lower. This is still another problem.
In order to overcome these problems, JP-A-No. 62-50354 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") illustrates an olefin soft resin composition obtained by hot-melting and mixing a mixture comprising (A) from 10 to 90 parts by weight of a polypropylene, (B) from 90 to 10 parts by weight of a halogenated butyl rubber (provided that (A)+(B)=100 parts by weight), (C) from 10 to 120 parts by weight of an olefin rubber and (D) from 5 to 120 parts by weight of a mineral oil softening agent in the presence of (E) a metal oxide and/or a metal chloride and then uniformly incorporating (F) a polyolefin into the said blend.
However, it has been found that the composition of JP-A-No. 62-5054 is still insufficient with regard to properties which are required for bumpers or the like parts of cars, though the composition has fairly good shapeability, impact-strength, coatability, softness and mechanical strength.
Accordingly, the object of the present invention is to provide a method of preparing a thermoplastic resin composition having more improved physical properties with respect to the above-mentioned points.